The present invention relates to arrays of fiber optic interferometric sensors and mechanisms for maximizing the signal to noise ratio in amplified sensor arrays that are time domain multiplexed.
Arrays of fiber optic interferometric sensors show promise in applications where size, electrical interference, and electromagnetic detection make electronic sensors impractical. Such interferometric sensors are capable of measuring a parameter (i.e., a measurand) with a very high dynamic range (e.g., 120 dB). Optical sensor arrays are formed by connecting a series of sensors using fiber optic lines. If each sensor in an array requires a dedicated fiber to carry the detection signal, the large number of fibers required quickly becomes unwieldy as the number of sensors increases. Thus, as the number of sensors in an optical array increases, time domain multiplexing (TDM) becomes necessary to maintain a low fiber count. Electrical and optical frequency domain multiplexing have been attempted, but they are unmanageable for arrays comprising hundreds of sensors. As a result, large sensor arrays are organized into long strings of sensors which perform TDM by returning information from sensors placed at discrete intervals. A typical passive sensor array using TDM is constructed in a ladder type configuration. This design has only a few fiber lines and permits a small deployment size. It is desirable to provide a multiplexing scheme which includes a large number of interferometric sensors in an array while preserving the high dynamic range of the sensors and maintains a high signal to noise ratio (SNR).
As shown in FIG. 1, a conventional passive optical array 10 using TDM is formed by using a splitter coupler 140 to couple a distribution bus 100 to a first end of an optical sensor 110. A second splitter coupler 142 couples a return bus 120 to a second end of the optical sensor 110. A detection signal is sent from a source (not shown) which is then partially coupled into the first sensor 110 in an array of n sensors. The remainder of the detection signal continues along the distribution bus to subsequent couplers, each coupling a fraction of the detection signal into successive sensors.
Each sensor modifies the optical signal coupled into it from the distribution bus 100 based on external (e.g., acoustic) perturbations to be detected. The perturbed signal is then coupled onto the return bus 120 by coupler 142. The return bus then transmits the perturbed signals out of the array for processing.
The basic principle of TDM is as follows. The length of the path that the optical signal takes from the source, along the distribution bus 100, through the coupler 140, the sensor 110, the coupler 142 and back along the return bus 120 is different for each sensor. Therefore, the return signals arrive at the detector at different time intervals depending on the path length. Sensors closer to the signal source have a shorter path than sensors near the end of the array. Thus, sensors near the source place the return signals on the return bus slightly earlier than sensors farther down the array. This assumes that the time delay through each of the sensors is relatively equal. The signals are then transmitted outside the array to be sequentially processed by other hardware to extract the sensed information. Because each of the return signals has different time delay based upon differing distances between the sensor and the source, it is possible to use optical signals in a pulsed form. Based on the foregoing, each sensor 110 returns a signal pulse which is slightly delayed from the signal pulse returned by the previous sensor, and therefore enables the various signal pulses to be temporally separated at the detector. To avoid overlap of the returned signals on the return bus 120 and at the detector, the pulse length and frequency of the optical signals are selected so that the return signals do not overlap on the return bus.
FIG. 8 illustrates a timing diagram for a sensor array employing TDM to multiplex the return signals onto the return bus for detection and processing. In time period 1, the signal source outputs a detection pulse of length xcfx84. The signal source then waits a period of TSystem before resetting itself and repeating the detection pulse (shown as time period 1xe2x80x2). Once the detection pulse has been issued from the signal source, it is split into each sensor. The signal from each sensor returns at a different time depending on each sensor""s respective distance from the signal source. The path lengths are chosen carefully so that the return signals are placed on the return bus at successive intervals with only a short intervening guard band (TGuardband) between the return signals to prevent signal overlap. Once the last sensor has returned a signal N to the detector, the system waits a reset period (TReset) and then restarts the process. The period TReset is selected to assure that the return pulse N from the last sensor arrives at the detector before the return pulse 1xe2x80x2 from the first sensor arrives in response to the second detection pulse. An exemplary period for TReset is approximately equal to TGuardband. Thus, the repetition period for TSystem is approximately Nxc3x97(xcfx84+TGuardband). For example, for a system having a path difference of approximately 8.2 meters between adjacent sensors, xcfx84 is selected to be approximately 40 nanoseconds and TGuardband is selected to be approximately 1 nanosecond. When the array is configured to include 300 sensors (i.e., N=300), then TSystem is approximately 12.3 microseconds. For this exemplary configuration, a repetition rate of approximately 80 kHz assures that the last return signal in response to a detection pulse does not overlap with the first return signal in response to the next detection pulse. Note that in FIG. 8 the time offset between the detection pulse and the first return pulse is not shown because the offset varies in accordance with the optical path length from the source to the first sensor, through the first sensor and back to the detector.
The advantage of TDM is that it allows simple interrogation techniques. No switching hardware is necessary, allowing a reduction in the cost and the size of the array. However, one of the problems with TDM is that it reduces the time each sensor is available for detection. If each sensor were given a dedicated fiber to report the result of its detections, it could provide a continuous stream of information. However, when TDM is implemented to reduce the number of fibers, no such continuous reporting is possible. The amount of time any one sensor is sampled is reduced to 1/N of a continuously sampled sensor. As the number of sensors grows, the amount of time and the frequency that any one sensor is sampled is further reduced.
The limited sampling time increases the significance of the signal to noise ratio (SNR). Since under TDM, a short sample is extrapolated to represent a much longer period (N times longer than its actual sample time), it is much more essential that each sample be interpreted correctly by the detector. Noise is a significant source of interpretation errors and therefore the SNR must be kept as high as possible with as little degradation of the SNR along the sensor array as possible. A high SNR reduces the number of interpretation errors by the detection system.
The detection signal experiences a significant loss as it propagates through the passive array. The sources of loss include, for example, (1) fiber loss, splice losses, and coupler insertion loss, (2) sensor loss, and (3) power splitting at each coupler on the distribution and return busses.
Simple splitting (loss item (3)), which is the method used to couple the optical sensor to the distribution and return buses, results in large losses and a severe degradation in the SNR. The amount of light in the detection signal coupled from the distribution bus into the sensor depends on the coupling ratio of the coupler. The coupling ratio approximately represents the fraction of light that is split into the sensors and approximately one minus the coupling ratio is the fraction of light that is passed down the distribution bus to the next coupler. A high coupling ratio results in more power being delivered to each sensor from the distribution bus, but also results in a smaller amount of power being available to downstream sensors. A low coupling ratio increases the power delivered downstream, but limits the power available to each sensor. Consequently, there is a value of the coupling ratio that maximizes the return power from the farthest sensors, as discussed below.
In an array containing N sensors, the power returning from the mth sensor decreases as m increases (where sensor m=1 is the closest sensor to the source). The exception is the signal from the last sensor number N, which does not experience a splitting loss since there is no coupling and the entire remainder of the signal passes through it. In the passive array shown in FIG. 1, the return signal is therefore the weakest for sensor number Nxe2x88x921. To achieve the best output signal-to-noise ratio in a passive optical array, the signal at the detector (1) should carry as much power as permitted by nonlinear effects in the fiber busses, and (2) should be shot noise limited (a condition in which quantum noise originating at the source of the signal dominates the noise characteristic of the signal).
Without specifying particular optical powers, integration times, pulse widths, repetition rates, and the optical filtering needed to determine an absolute output SNR, the following equations define a system noise figure component which can be used to compare different array configurations. The noise figure of interest is the input source SNR divided by the output SNR for the worst sensor in the array (the Nxe2x88x921st sensor). The system noise figure (NF) is defined as:                               NF          system                =                              SNR            intoarray                                SNR            outworstsensor                                              (        1        )            
This definition is consistent with the classical definition of amplifier noise, but is used here to describe the whole system as an amplification-loss transformation.
In order to determine the noise figure of the system, the losses associated with the various elements of the system (e.g., splicing losses, splitting losses, coupler losses, etc.) must be calculated. These losses (L) are considered in dB""s (negative dB""s in particular). The losses can also be considered in terms of transmissions. For example, axe2x88x923 dB loss is a 50% transmission, and axe2x88x9210 dB loss is a 10% transmission. It is assumed that each sensor imparts the same loss Ls to the signal, and the excess loss due to splices and coupler insertion is the same for all coupler segments and is equal to Lx. When all couplers exhibit the same coupling ratio C, it can then be shown that the power returning to the detector from sensor number m is:
Pm=Pinto array(1xe2x88x92C)2mxe2x88x922Lx2mxe2x88x922C2Ls for m less than Nxe2x80x83xe2x80x83(2)
For the embodiment shown in FIG. 1, the sensor N receives more optical power than the sensor Nxe2x88x921 because the sensor N is connected directly to the distribution fiber rather than being coupled. The power for the sensor N is:
PN=Pinto array(1xe2x88x92C)2Nxe2x88x922Lx2Nxe2x88x922Lsxe2x80x83xe2x80x83(3)
Thus the returning power is lowest for sensor number Nxe2x88x921. From Equation 2, this power depends on the coupling ratio C and is at a maximum when:                     C        =                  1                      N            -            1                                              (        4        )            
Using Equations 1 and 2, and assuming an optimized coupling ratio (Equation 4), the noise figure for the worst sensor is:                               NF          passive                =                                            (                              N                -                1                            )                                                      2                ⁢                N                            -              2                                                          L              s                        ⁢                                                            L                  x                                                            2                      ⁢                      N                                        -                    4                                                  ⁡                                  (                                      N                    -                    2                                    )                                                                              2                  ⁢                  N                                -                4                                                                        (        5        )            
FIG. 4b shows the noise figure for the optimized passive array (solid curve) as the number of sensors increases. The sensor loss is assumed to be Ls=6 dB, and is consistent with current sensor technology. The excess loss is assumed to be Lx=0.2 dB per coupler segment. FIG. 4b shows that the noise figure level rises rapidly as the number of sensors is increased, revealing the limitations of the passive array configuration.
In order to obtain longer sensor arrays, a passive optical array must accept a reduction in the power available to each individual sensor, and therefore a degradation in the SNR results. With these constraints in mind, maximizing the SNR in TDM sensor arrays has been difficult. One solution is to increase the power in the optical source, which will, under shot-noise limited conditions, increase the SNR of all return signals. However, the maximum power the distribution bus can transmit is limited by nonlinear effects in the optical fiber. A passive array design is therefore limited in its ability to compensate for the low power coupled into each sensor by raising the initial power of the optical source.
Since the SNR is a large factor in the performance of a TDM optical sensor array, if the levels of noise in the resulting detection signal are high, the limits of current sensor technology cannot be approached and the benefits of highly sensitive sensors can never be exploited. For this reason, the architecture and design parameters of sensor arrays must be selected to minimize the SNR degradation due to splitting, other fiber losses and the presence of other noise. The present invention significantly improves the SNR in a passive optical array by adding optical amplifiers between the couplers to compensate for the coupler splitting losses.
In one advantageous embodiment of the present invention, optical amplifiers are inserted between the couplers along the signal path. The gain of the amplifiers is designed to compensate for the losses due to the previous coupler and other fiber losses. In this way, the overall SNR can be maintained without significant degradation as the number of sensors in the array increases. In a first aspect of the present invention, the amplifiers are located along the distribution and return buses directly after the couplers (except for the last sensor). In a second aspect of the present invention, the amplifiers are located directly before the couplers.
In one embodiment, the optical amplifiers comprise short lengths of erbium-doped fiber spliced into the distribution and return buses. Inexpensive pump sources can be used to pump the amplifiers from one or both ends of the array at 1480 nm or 980 nm for Er-doped fiber and at 1060 nm for Er/Yb-doped fiber.
Improvements can be made to the SNR when the distribution bus coupling ratios are set at optimal values. The value of the optimal coupling ratio depends upon the amplifier configuration, the excess loss and other configuration parameters.
Additional benefits can be achieved by grouping sensors into parallel configurations along the distribution and return buses. In this way, the number of sensors can be increased significantly without a corresponding increase in the number of amplifiers required. The parallel grouping of multiple sensors can increase the sensor density without a corresponding increase in the number of amplifiers or couplers. This design can improve the SNR by reducing the overall number of amplifiers and couplers, thereby reducing amplifier spontaneous emission noise and coupling losses. Also, the pump power requirements are reduced. This aspect of the present invention also permits smaller sized arrays for an equivalent number of sensors.
One aspect of the present invention is an optical sensor architecture which comprises a plurality of sensors which receive an optical signal and which output perturbed optical signals. A distribution bus is coupled to each sensor to distribute the optical signal to each sensor. A return bus is coupled to each sensor to receive the perturbed optical signal from each sensor to be included as a portion of the return signal. A plurality of first optical amplifiers are distributed at selected positions along the length of the distribution bus to maintain the power of the distributed optical signal at a selected level. A plurality of second optical amplifiers are distributed at selected positions along the length of the return bus to maintain the power of the perturbed optical signals in the return signal.
Another aspect of the present invention is an optical sensor architecture which comprises a plurality of sensor groups. Each sensor group comprises at least one sensor which receives an optical signal and which outputs a perturbed optical signal. A distribution bus is coupled to each sensor group to distribute the optical signal to each sensor group. A return bus is coupled to each sensor group to receive the perturbed optical signal from each sensor group. A plurality of first optical amplifiers are distributed at selected positions along the length of the distribution bus to maintain the power of the optical signal at an adequate level for each sensor group. A plurality of second optical amplifiers are distributed at selected positions along the length of the return bus to maintain the power of the perturbed optical signals on the return bus.
A further aspect of the present invention is an optical sensor architecture which comprises a plurality of means for sensing a parameter; means for distributing a first optical signal to each of the means for sensing; means for returning a second optical signal from each of the means for sensing; a plurality of means for amplifying the first optical signal spaced along the means for distributing; and a plurality of means for amplifying the second optical signal spaced along the means for returning.
A further aspect of the present invention is a method for reducing a noise figure level in a signal returning from a sensor architecture to generate an optical output. The method uses a plurality of sensors to generate output signals. An optical signal is transmitted through a distribution bus coupled to each sensor. The output signal from each sensor is coupled into a return signal carried via a return bus coupled to each sensor. The optical and return signals are amplified at multiple stages along the distribution and the return buses to increase a signal to noise ratio within the sensor architecture.
A further aspect of the present invention is a method for optimizing an array of optical sensors. The method provides an array of optical sensors positioned between a distribution fiber which propagates an input optical signal from a source and a return fiber which returns perturbed optical signals to a detector. Each optical sensor is coupled to the distribution fiber by a respective input coupler and coupled to the return fiber by a respective output coupler. A plurality of amplifiers are interposed at selected locations on the input distribution fiber and the return fiber. The amplifiers compensate for losses in the array. Coupling ratios are selected for the couplers and gains are selected for the amplifiers to optimize a system noise figure. The system noise figure is the ratio of a signal to noise ratio of the input optical signal to a signal to noise ratio of an optical signal in a sensor having a lowest signal to noise ratio.
A still further aspect of the present invention is a method for optimizing an array of optical sensors. The method provides an array of optical sensors coupled to an optical fiber by a plurality of couplers. An optical signal propagating in the optical fiber is amplified by a plurality of amplifiers to compensate for losses in the array. Coupling ratios are selected for the couplers and gains are selected for the amplifiers to optimize a system noise figure. The system noise figure is the ratio of a signal to noise ratio of the input optical signal to a signal to noise ratio of an optical signal in a sensor having a lowest signal to noise ratio.
A still further aspect of the present invention is an optical sensor architecture. The architecture comprises a plurality of sensors which receive an input optical signal and which output perturbed optical signals in response to. a sensed parameter. At least one optical fiber distributes an optical signal to each sensor and returns a perturbed optical signal from each sensor. A plurality of optical amplifiers distributed at selected positions along the length of the at least one optical fiber to maintain the power of the distributed optical signal and returned perturbed optical signals at selected levels.
Another aspect of the present invention is an optical sensor array architecture which comprises a distribution bus which receives and distributes an optical input signal. The distribution bus propagates a distribution bus pump signal. A return bus receives a plurality of optical return signals and provides the optical return signals as output signals. The return bus propagates a return bus pump signal. A plurality of rungs are coupled between the distribution bus and the return bus. Each of the rungs comprises at least one sensor which receives a respective portion of the optical input signal and which generates one of the optical return signals. A plurality of input optical amplifiers in the distribution bus are responsive to the distribution bus pump signal. The input optical amplifiers amplify the optical input signal and have gains which maintain the optical input signal at a selected signal level for each of the rungs. A plurality of output optical amplifiers in the return bus are responsive to the return bus pump signal. The output optical amplifiers amplify the return signals generated by the sensors in the rungs and have gains which substantially equalize the magnitudes of the optical return signals. The gains of the amplifiers are typically greater when pumped by greater pump energy. Also preferably, the distribution bus pump signal and the return bus pump signal enter respective ends of the distribution bus and the return bus. The distribution pump signal may cause unequal pumping of the input optical amplifiers and differences in the respective gains of the input optical amplifiers. The return bus pump signal may cause unequal pumping of the output optical amplifiers and differences in the respective gains of the output optical amplifiers. The input optical amplifiers, the output optical amplifiers and the rungs are located such that the architecture defines a plurality of optical paths which include different combinations of the input optical amplifiers and the output optical amplifiers which have respective cumulative gains. The input optical amplifiers and the output optical amplifiers have gains selected such that differences in the cumulative gains between the optical paths are reduced, thereby reducing the noise figure of the architecture. The amplifiers are preferably positioned along the buses such that the optical paths include an equal number of amplifiers. The respective gains of the amplifiers are preferably adjusted to compensate for losses within the optical sensor architecture to maintain near unity transmission along the buses.
Another aspect of the present invention is a method of reducing the noise figure of an optical sensor architecture. The method comprises providing distribution and return buses through which pump energy propagates. The pump energy provides gain to optical amplifiers positioned along the distribution and return buses. The method further includes providing a plurality of rungs and a plurality of couplers. The couplers connect each of the rungs to the distribution and return buses. Each of the rungs comprises at least one sensor which receives a respective portion of an optical input signal launched into the distribution bus. The sensors generate respective optical return signals which enter the return bus. The method further comprises selecting the number of the rungs and the number of sensors in each rung to provide a total number of the sensors approximately equal to a desired number of total sensors. The number of rungs and the numbers of sensors in the rungs are selected to reduce the noise figure of the optical sensor architecture. In certain embodiments according to the method, the number of the rungs and the numbers of the sensors in the rungs are selected to reduce, but not minimize, the noise figure, so that the distribution and return pump power requirements are also reduced. Also, in certain embodiments, the fraction of the optical input signal coupled into the rungs by the couplers in the distribution bus is selected to reduce the noise figure of the optical sensor architecture for certain levels of optical input signal and distribution and return pump signals.
Another aspect of the present invention is a method of reducing the noise figure of an optical sensor architecture. The method comprises providing distribution and return buses through which pump energy propagates. The pump energy provides gain to optical amplifiers positioned along the distribution and return buses. The method further comprises providing a plurality of rungs and a plurality of couplers. The couplers connect each of the rungs to the distribution and return buses. Each of the rungs comprises at least one sensor which receives a respective portion of an optical input signal launched into the distribution bus. The sensors generate respective optical return signals which enter the return bus. The method further comprises selecting respective fractions of the optical input signal coupled into the rungs by the couplers in the distribution bus and respective fractions of the optical return signals coupled into the return bus by the couplers in the return bus to reduce the noise figure of the optical sensor architecture for a total number of the sensors approximately equal to a desired number of total sensors.
Another aspect of the present invention is an optical sensor architecture which comprises a distribution bus and a return bus, both of which propagate pump energy. The pump energy provides gain to optical amplifiers positioned along the distribution and return buses. The architecture includes a plurality of rungs and a plurality of couplers. The couplers connect each of the rungs to the distribution and return buses. Each of the rungs comprises at least one sensor which receives a respective portion of an optical input signal launched into the distribution bus. The sensors generate respective optical return signals which enter the return bus. The number of the rungs and the number of sensors in each rung provide a total number of the sensors approximately equal to a desired number of total sensors. The number of rungs and the numbers of sensors in the rungs are selected to reduce the noise figure of the optical sensor architecture.
Another aspect of the present invention is an optical sensor architecture which comprises a distribution bus and a return bus, both of which propagate pump energy. The pump energy provides gain to optical amplifiers positioned along the distribution and return buses. A plurality of rungs and a plurality of couplers connect each of the rungs to the distribution and return buses. Each of the rungs comprises at least one sensor which receives a respective portion of an optical input signal launched into the distribution bus. The sensors generate respective optical return signals which enter the return bus. The respective fractions of the optical input signal coupled into the rungs by the couplers in the distribution bus and the respective fractions of the optical return signals coupled into the return bus by the couplers in the return bus are selected to reduce the noise figure of the optical sensor architecture for a total number of the sensors approximately equal to a desired number of total sensors.